This invention pertains to the fabrication of low density, foamed articles of normally solid thermoplastic polymer resins and more particularly to the use of non-tacky hydrogels formulated from a mixture of resin, organic solvent, and water for this purpose.
Solvent plasticization of polymers is a known technique of producing low density polymer foams at low processing temperatures. A major problem in fabricating low density, foamed polymer resins from solvent plasticized polymers in gel form is the high adhesion the gel exerts on substrates which it may contact. This adhesion to substrate creates problems in handling and transferring of the plasticized gel into molds which form the foamed shaped. This problem also exists with polymer pellets which are highly plasticized.
There is a continuing effort to discover ways of handling these tacky polymer gels because of the cost saving which accompanies the lowering of the density of the polymer foams produced from them. Although the physical properties of polymers are generally reduced when the polymers are plasticized, due to the lowering of the second order transition temperatures (Tg), this deficiency is negatived when fugitive organic solvents are used to produce the polymer gels.
Increasing the solvent content of the polymeric gel increases the mold fill by increasing the gel's plasticity. However, as the plasticity is increased the resultant adhesion to substrates of the gel is also increased.
Another problem associated with highly solvent plasticized polymers which contain low boiling point solvents is the containment of the solvent within the gelled polymer for extended periods of time subsequent to the final foaming of the shaped article.